1. Field of the Invention
This invention relates to a stabilized current generator, particularly suitable for being built-in in integrated circuits of the MOS (Metal-Oxide-Semiconductor) type.
In integrated circuits, the need often arises to generate, inside the circuit itself, a current of a desired value. A typical example is represented by the biasing stage of an operational amplifier.
It is known to use, for this purpose, current generators such as the Wilson generator, or the cascode generator ("Basic MOS Operational Amplifier Design--An Overview", Section IIc, by P. R. Gray, in Analog MOS Integrated Circuits, IEEE Press, New York, 1980, page 28; and "Design Considerations in Single-Channel MOS Analog Integrated Circuits--A Tutorial", Section II, by Y. P. Tsividis, in IEEE Journal of Solid-State Circuits, vol. SC-13, No. 3, June 1978, p. 383).
Such generators, however, are only suitable for applications in which a high accuracy of the value of the current is not required, particularly when the current variations due to variations of the electric and physical parameters of the integrated circuit (such as conduction factors and threshold voltages of transistors, resistance per square of the resistive layers, etc.) and of the environmental and operating conditions of the circuit itself (e.g. supply voltages, temperature, etc.) do not pose a problem.
When, however, a very accurate value of generated current is required, for example within .+-.10% of the rated value, also taking into account the variations of the electric and physical parameters of the manufacturing process of the integrated circuit, and furthermore it is required that said value is substantially independent from the operating conditions, in particular from the value of the supply voltage and from the temperature, the above mentioned generators are no longer satisfactory.
It is thus known in these cases to use current mirror generators, in which the driving current is obtained starting from a reference voltage (which is usually available with a very high accuracy on the integrated circuit). The obvious way to obtain such a driving current would be to apply said reference voltage across a resistor having a very accurate value. Since implementing a resistor having an accurate and constant value is difficult in MOS-type circuits, where, on the other hand, it is easy to provide capacitive elements having a sufficiently accurate and constant value, it is also known to achieve an equivalent result by employing circuit means using switched capacitors, the switching being performed by electronic switches controlled by a clock signal (see, e.g., "Sampled Analog Filtering Using Switched Capacitors as Resistor Equivalents", by J. T. Caves, M. A. Copeland, C. F. Rahim and S. D. Rosenbaum in IEEE Journal of Solid-State Circuits, vol SC-12, No. 6, December 1977).